Output voltage stabilizer for switching mode power supply

ABSTRACT

There is provided an output voltage stabilizer for stabilizing an output voltage of a switching mode power supply by elimination power bounce appearing on the output voltage of the switching mode power supply. The output voltage stabilizer mainly includes a voltage comparator that compares a DC voltage with a reference voltage and in response thereto generates an output signal, and a control switch that is biased according to the output signal of the voltage comparator and provides the power supply a drive signal to stabilize an output voltage of the power supply according to an on/off state of the control switch.

FIELD OF THE INVENTION

[0001] The present invention is generally related to a voltagestabilizer for smoothly turning on and turning off a switching modepower supply, and more particularly, the present invention is related toa voltage stabilizer that is used to stabilize the output voltage of aswitching mode power supply by the elimination of power bounce appearingon the output voltage of a power supply.

BACKGROUND OF THE INVENTION

[0002] A power supply is generally used to convert commerciallyavailable alternating current power to direct current power for use byan ordinary electronic appliance. The conversion technique used in apower supply for a personal computer is based on the switching operationof switching devices to provide DC output voltage of multiplepredetermined levels. This type of power supply is generally termed asswitching regulator or switching mode power supply (SMPS).

[0003] However, the implication of power quality of a switching modepower supply always depends on its capability to exclude abruptdisturbance or maintain power conversion efficiency. A common problemthat would cause the instability of the power quality of a power supplyis due to voltage sag or voltage dip that is caused by an accidentoccurring between adjacent feed lines or voltage buses. The voltage saggenerally means that the root-mean-square value (rms value) of voltageis dropped below its nominal value by 10% to 90%, and it would last fora prolonged period, for example, several seconds. The generation ofvoltage sag in an AC power source would greatly lessen the reliabilityof a power supply.

[0004] In a power supply system, brownout condition signifies asignificant degradation of the primary power source. The danger ofbrownout condition is that it can repeat several times consecutively andis generally followed by a follow-on surge that can be several times ofthe voltage that a switching mode power supply is supposed to take in.It has been discovered in numerous brownout tests for switching modepower supply that in case the input voltage is abruptly dropped belowits nominal value by about 50%, the output voltage is still underregulation. However, in case the input voltage is abruptly dropped belowits nominal value by 80%, the output voltage will run out of regulationand start to bounce violently. The unstable output voltage generated bythe power supply is likely to damage other electronic circuit powered bythe power supply.

[0005] In addition to the brownout problem that is induced as the inputvoltage abruptly dropped below its nominal value by a predeterminedamount, when the power supply is started up, a surge current may beinduced at the input terminals of the power supply. Therefore the outputvoltage of power supply may overshoot upon start-up and sparks may begenerated. To limit the current surge from the input terminals of powersupply upon start-up and provide a simple way to softly start the powersupply, a device featuring with soft-start function is required toensure a soft start-up of the power supply.

[0006] In view of foregoing problems, what is needed is an outputvoltage stabilizer that can stabilize the output voltage of power supplyduring brownout stage and/or soft-start process. The present inventioncan satisfy these needs.

SUMMARY OF THE INVENTION

[0007] A first object of the present invention is to provide an outputvoltage stabilizer for a switching mode power supply that can limit theoutput voltage from bounce when an abrupt voltage sag is occurred to theinput AC power of the power supply.

[0008] A second object of the present invention is to provide an outputvoltage stabilizer for a switching mode power supply that enable theswitching mode power supply to turn on smoothly.

[0009] Specifically, the present invention contrives an output voltagestabilizer for a switching mode power supply, comprising a voltagecomparator that compares a DC voltage with a reference voltage and inresponse thereto generates an output signal, and a control switch thatis biased according to the output signal of the voltage comparator andprovides the switching mode power supply a drive signal to stabilize anoutput voltage of the switching mode power supply according to an on/offstate of the control switch.

[0010] In a boarder aspect of the present invention, a switching modepower supply is disclosed which is comprised of a rectifier forconverting AC power received from an AC power source into rectified DCpower, a switching transformer coupled to the rectifier and including aprimary winding and a secondary winding for receiving the rectified DCpower and generating AC voltage across the secondary winding through theprimary winding, a switching device coupled to the primary winding ofthe switching transformer for converting a rectified DC power into ACpower by on/off operations, an output portion coupled to the secondarywinding of the switching transformer for providing an output voltage ofa predetermined value, a switching control circuit which outputsswitching control pulse signals to control on/off operations of theswitching device, and an output voltage stabilizer coupled between therectifier and the switching control circuit which compares the rectifieddirect current power generated by the rectifier with a reference voltageand in response thereto provides a drive signal to drive the switchingcontrol circuit to stabilize the output voltage of power supply.

[0011] The features and advantages of the present invention will becomeapparent through the following descriptions with reference to thedrawings presented herein, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a circuit diagram showing a switching mode power supplyin conjunction with the output voltage stabilizer incorporated thereinin accordance with the present invention; and

[0013]FIG. 2 shows the detailed circuitry of the output voltagestabilizer in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] The exemplary embodiments of the present invention will now bedescribed in detail with reference to the following preferredembodiments. However, it is to be noted that various modifications canbe made on the basis of the present invention, without departing fromthe spirit and scope of the present invention as to be encompassedwithin the present invention.

[0015] The best mode for carrying out the present invention is depictedas follows. FIG. 1 shows the circuit topology of the switching modepower supply according to an embodiment of the present invention. Asshown in FIG. 1, reference numeral 11 denotes an EMI filter thatattenuates noise signals appearing between power lines extending from anAC power source, a reference numeral 12 denotes a full-wave rectifierthat rectifies the input AC voltage Vac into a DC voltage Vdc, areference numeral 13 denotes a switching device such as a power MOSFETthat applies the DC voltage Vdc from the full-wave rectifier 12 to theprimary winding side of a switching transformer 14 to generate ahigh-frequency AC power therein, a reference numeral 14 denotes aswitching transformer which generates AC voltage across its secondarywinding as the switch device 13 turns on and off, a reference numeral 15denotes a switching control circuit for outputting switching controlpulse signals from driving pulse pin (DRV pin) to control the on/offoperations of the switch device 13, a reference numeral 16 denotes anoutput portion for providing a DC output voltage Vo at output terminals.In addition, the switching transformer 14 includes an auxiliary windingNa that provides supply power for the switching control circuit 15through the voltage supply pin VCC of the switching control circuit 15.

[0016] The switching control circuit 15 may use NCP1222 PWM current-modecontroller manufactured by On Semiconductor Corporation, and acompilation of its pin function description is tabulated in Table 1 forreference.

[0017] Table 1 is a compilation of pin function description of NCP1222PWM current-mode controller manufactured by On SemiconductorCorporation. Pin No. Pin Name Function Description 1 Adj Adjust theskipping peak This pin is used to adjust current the level at which thecycle skipping process takes place 2 FB Set the peak current Byconnecting an setpoint optocoupler to this pin, the peak currentsetpoint is adjusted according to the output power demand 3 CS Currentsense input This in sense the primary current and routes it to theinternal comparator via an L.E.B. 4 GND The IC ground 5 DRV Drivingpulses The driver's output to an external MOSFET 6 VCC Supplies the ICThis pin is connected to an external bulk capacitor of typically 10 μF 7NC No connection This un-connected pin ensures adequate creepagedistance 8 HV Generates the VCC from Connected to the high- line voltagerail, this pin injects a constant current into the VCC bulk capacitor

[0018] Still referring to FIG. 1, the feature of the present inventionis highlighted by an output voltage stabilizer 17. As shown in FIG. 1,an output voltage stabilizer 17 is incorporated in the switching modepower supply and coupled between an output of the rectifier 12 and afeedback pin (FB pin) of the switching control circuit 15. The mainpurpose of the incorporation of output voltage stabilizer 17 in aswitching mode power supply is to drive the feedback pin of theswitching control circuit 15 to a logic state so as to regulate theoutput voltage Vo without bounce, if the input AC voltage Vac isundergoing a voltage sag process or is experiencing an initiationprocess. Referring to FIG. 2, a detailed circuitry that looks inside theoutput voltage stabilizer 17 is shown. The output voltage stabilizer 17according to the exemplary embodiment of the present inventionprincipally consists of a voltage comparator 21 that compares the DCcomponent Vdc of the input AC voltage Vac generated by the full-waverectifier 12 with a reference signal Vref, and a control switch 22 suchas a bipolar junction transistor (BJT) switch which receives the outputsignal from the voltage comparator 21 and responsive to the outputsignal of the voltage comparator 21 to output a drive signal to drivethe feedback pin of the switching control circuit 15.

[0019] More preferably, the voltage comparator 21 may use LM358single-supply dual-operational amplifier, also manufactured by OnSemiconductor Corporation.

[0020] The operation of the output voltage stabilizer 17 according tothe present invention will be explained as follows in reference to FIGS.1, 2 and Table 1.

[0021] Assume that the input AC voltage Vac slowly increases from 0 toVac, the DC component Vdc of input AC voltage Vac generated by thefull-wave rectifier 12 will increase as well. Also, the auxiliarywinding Na of the switching transformer 14 starts to release supplyvoltage to the switching control circuit 15 through voltage supply pinVCC. The supply voltage pin VCC for the switching control circuit 15also applies to the eighth pin of the voltage comparator 22 of theoutput voltage stabilizer 17. Moreover, the reference voltage Vref usedin the voltage comparator 21 is generated by dividing the supply voltageVCC by a fraction determined by the voltage divider R29, R30.

[0022] When the supply voltage VCC is higher than the voltage of thezener diode ZD1, the reference voltage Vref applied to the non-invertingterminal of the voltage comparator 21 will become stable. At thismoment, the reference voltage Vref applied to the non-inverting terminalof the voltage comparator 21 is higher than the voltage applied to theinverting terminal of the voltage comparator 21, so that the voltagecomparator sends an output signal with a logic high state to the baseterminal of the BJT switch 22. The BJT switch 22 will then be biased toturn on, and thus send a drive signal to the feedback pin of theswitching control circuit 15 to conduct the feedback pin of theswitching control circuit 15 to ground. When the input AC voltage Vacincreases to reach the point that voltage applied to the invertingterminal of the voltage comparator 21 is higher than that applied to thenon-inverting terminal, the logic state of the output signal of thevoltage comparator 21 becomes low and the BJT switch 22 is biased toturn off. At this moment the state of feedback pin of the switchingcontrol circuit 15 becomes high and the switching control circuit 15starts to output switching control pulse signals through drive pulse pinDRV to drive switch device 13. In the mean time, the output voltage ofthe power supply start to rise up and is regulated without bounce.

[0023] In a second aspect of the present invention, a brownout test isperformed to the switching mode power supply of the present invention.Under brownout condition, the input AC voltage Vac is dropped. When Vacis continuously dropped down to the level that the voltage applied tothe inverting terminal of the voltage comparator 21 is lower than thatapplied to the non-inverting terminal of the voltage comparator 21, thestate of the output signal of voltage comparator 21 goes high and theBJT switch 22 is biased to turn on. The feedback pin of the switchingcontrol circuit 15 will be conducted to ground again to shutdown theswitching control circuit 15. At this moment the output voltage Vo willdrop to 0V without bounce.

[0024] According to the above statements, the use of the output voltagestabilizer 17 in a switching mode power supply is to clamp the state ofthe feedback pin of switching control circuit 15 to a low state when theinput AC voltage Vac is within a certain range of voltage level, so asto ensure a complete transfer of energy from the primary side ofswitching transformer 14 to the output portion 16 circuit located at thesecondary side of the switching transformer 14. With the introduction ofthe output voltage stabilizer of the present invention into theswitching mode power supply, the signal bounce occurring to the outputvoltage of the power supply can be eliminated, and thus a stable outputvoltage can be attained without effort.

[0025] It is clear in virtue of the above descriptions that the basicprinciple of the present invention is to use an output voltagestabilizer to clamp the voltage level of feedback pin of switchingcontrol circuit to a low state when the input AC voltage Vac is within acertain range, such that a complete transfer of energy from the primaryside of switching transformer 14 to the output portion 16 can be ensuredand the output voltage can be stabilized. Whether the switching modepower supply is under either brownout condition or start-up condition,its output voltage can be stabilized by controlling the feedback amountthat is transferred to the switching control circuit 15.

[0026] Although the invention has been described and illustrated indetail, it is to be clearly understood that the same is by the way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

What is claimed is:
 1. An output voltage stabilizer for a power supply,comprising: a voltage comparator which compares a direct current voltagewith a reference voltage and in response thereto generates an outputsignal; and a control switch which is biased according to said outputsignal and provides said power supply a drive signal to stabilize anoutput voltage of said power supply according to an on/off state of saidcontrol switch.
 2. The output voltage stabilizer as claimed in claim 1wherein said voltage comparator comprises a dual-operational amplifier.3. The output voltage stabilizer as claimed in claim 1 wherein saidcontrol switch comprises a bipolar junction transistor.
 4. A powersupply comprising: a rectifier for converting alternating current powerreceived from an alternating current power source into direct currentpower; a switching transformer coupled to said rectifier and including aprimary winding and a secondary winding for receiving said directcurrent power and generating alternating current voltage across saidsecondary winding through said primary winding; a switching devicecoupled to said primary winding of said switching transformer forconverting a rectified direct current power into alternating currentpower by on/off operations; an output portion coupled to said secondarywinding of said switching transformer for providing an output voltage ofa predetermined value; a switching control circuit which outputsswitching control pulse signals to control said on/off operations ofsaid switching device; and an output voltage stabilizer coupled betweensaid rectifier and said switching control circuit which compares saiddirect current power with a reference voltage and in response theretoprovides a drive signal to drive said switching control circuit tostabilize said output voltage.
 5. The power supply as claimed in claim 4wherein said power supply further comprising an electromagnetic filtercoupled to said rectifier for reducing noise signal appearing betweenpower lines extending from an alternating current power source.
 6. Thepower supply as claimed in claim 4 wherein said switching transformerfurther comprises an auxiliary winding.
 7. The power supply as claimedin claim 6 wherein said switching control circuit is powered throughsaid auxiliary winding of said switching transformer.
 8. The powersupply as claimed in claim 4 wherein said switch device comprises apower MOSFET.
 9. The power supply as claimed in claim 4 wherein saidoutput voltage stabilizer further comprises: a voltage comparator whichcompares said rectified direct current voltage with a reference voltageand in response thereto generates an output signal; and a control switchwhich is biased according to said output signal and provides saidswitching control circuit a drive signal to regulate said output voltageof said power supply according to an on/off state of said controlswitch.
 10. The power supply as claimed in claim 9 wherein said voltagecomparator comprises a dual-operational amplifier.
 11. The power supplyas claimed in claim 9 wherein said control switch comprises a bipolarjunction transistor.